The present invention relates to a steel wire having a high fatigue strength best suited to spring, PC steel wire and so on, and to a method of manufacturing such a steel wire. More specially, the invention relates to such a steel wire having an excellent heat resistance or delayed fracture properties as well and to a method of manufacturing such a steel wire.
Spring steel wires containing 0.6-0.8 mass % of C, 0.15-0.35 mass % of Si, and 0.3-0.9 mass % of Mn are known in the art. Such a steel wire is manufactured by being processed through steps of rollingxe2x86x92patenting (heating for xcex3-phase transitionxe2x86x92isothermal transformation)xe2x86x92wire drawingxe2x86x92(coiling)xe2x86x92strain relief annealing (for example, at 300xc2x130xc2x0 C.).
However, it is a well-known fact that such a type of steel wire obtained by drawing a pearlite steel (generally called a piano wire or hard drawn steel wire: hereinafter shall be generically referred to as a piano wire) has a relatively low heat resistance.
Therefore, in high temperature environments where a permanent set resistance is required, quenched and tempered steel wires such as heat-resistant piano wires having a high Si content and oil tempered wires of Sixe2x80x94Cr steel (hereinafter shall be referred to as OT wire) have been used. Working environments requiring a heat resistance include a case of galvanizing a steel wire, for example, and it is customary to add Si to the steel in order to prevent or retard a decrease in strength in the course of the galvanization process.
In addition, it has been proposed that a steel wire having a high strength and toughness can be obtained by finely dividing cementite into microcrystals of a nano-order size. (Japanese Provisional Publication NO. 120407/96.)
However, the aforementioned prior arts have had a number of problems as follows:
(1) While important properties for steel wires include: a) high tensile strength, b) high toughness, and c) high fatigue strength, a high tensile strength is not necessarily compatible with a high fatigue strength in those steel wires to be processed through drawing. Generally, the tensile strength of a steel wire increases with its working ratio of drawing (reduction ratio). In addition, a fatigue strength cannot be increased without a comparatively high tensile strength. However, increasing the working ratio will result in increased micro defects of the material through plastic working, and such micro defects, when concentrated, will act as origins of earlier occurring fatigue fractures.
(2) A heat-resistant piano wire generally has a high Cr content and takes a longer time for its heat treatment (patenting), resulting in a lower productivity.
(3) The use of a heat-resistant piano wire as a steel wire to be galvanized or otherwise exposed to heat (at about 450xc2x0 C. for about 30 seconds) is intended to limit or retard a decrease in strength, but not to provide a thermal permanent set resistance at about 200xc2x0 C. or so. It is known in a parallel wire and the like steel wires that heat resistance is improved by increasing the Si content. In this respect, however, the purpose of using steel wires having a good heat resistance varies with their specific uses, the heat resistance for the case of parallel wire fundamentally aims at limiting the change in tensile strength of the wire small when subjected to galvanization. On the other hand, in the case of automobile engine valve springs exposed to intense heat in operation or automobile torsion bars heated to at about 200xc2x0 C. when car bodies are bake-finished, important considerations include keeping the permanent set in the temperature range of about 100-200xc2x0 C. small and at the same time providing desired fatigue properties. Thus, simply applying a chemical composition of such a parallel wire to a spring wire cannot bring forth satisfactory properties sufficient for a spring material. That is to say, while the Si addition in a parallel wire is reportedly said to be effective in improving its fatigue properties, this is mere a story of fatigue under repeated tension, which differs essentially from the fatigue properties required for a spring material. A decrease in surface hardness greatly affects the fatigue properties in a spring steel wire having a high Si content, although its influence on the fatigue properties is small in a parallel wire.
(4) As for a heat-resistant piano wire, even the delayed fracture properties important for a spring are not usually taken into consideration. Steel wire may sometimes be subjected to cationic coating and the like processing for an anticorrosion purpose, and delayed fracture may be caused then if hydrogen gets into it the steel wire. Especially, in a spring steel wire, delayed fracture properties to torsion stress are important, but such delayed fracture properties has hardly been taken account of so far.
(5) OT wire is expensive. While a steel wire superior in both heat resistance and fatigue strength can be obtained by applying quenching and tempering in the final stage of the steel wire manufacture, such a quenching and tempering process adds to the cost.
Accordingly, an object of the present invention is to provide a steel wire having a high heat resistance (particularly at around 200xc2x0 C.) and a high fatigue strength that can be produced without applying a quenching and tempering process, namely, produced through a drawing process and a method of manufacturing such a steel wire.
Another object of the present invention is to provide a steel wire having superior delayed fracture properties in addition to the heat resistance.
A further object of the present invention is to provide a steel wire having superior fatigue properties that can be achieved by improving its material strength and at the same time by optimally minimizing the origins of fatigue fracture and a method of manufacturing such a steel wire.
The present invention comprises the following features [1], [2], [3] and [4]:
[1] The present invention provides a steel wire comprising a pearlite structure containing 0.7-1.0 mass % of C and 0.5-1.5 mass % of Si, wherein in the cross section of the steel wire the difference in average hardness between a region up to 50 xcexcm from the surface thereof and a more deeper region is within 50 in micro-Vickers hardness. This steel wire has a high heat resistance and fatigue strength, and is particularly suited for spring steel wire.
Preferably, the steel wire may further contain 0.03-0.1 mass % of Mo. Further, it may contain 0.3-0.9 mass % or less Mn and/or 0.2 mass % or less Cr. For providing a sufficient fatigue strength, this steel wire preferably has a tensile strength above 1800 N/mm2.
Here, it is desirable that in the metal structure of the above steel wire the proeutectoid (granular) ferrite content is below 5 vol. %. Further, as to the shape of cementite particles constituting the pearlite structure, it is desirable that at least 80 vol. % of the cementite particles satisfy the following formula (1):
L/txe2x89xa75xe2x80x83xe2x80x83(1)
where t is the thickness and L is the length of the cementite particles.
Requirements for achieving such metal structures are that given the following formula (2):
10xc3x97(C(mass %)xe2x88x920.76)xe2x88x92Si(mass %)+5xc3x97Cr(mass %)=Txe2x80x83xe2x80x83(2)
the cooling rate V (xc2x0 C./sec.) after heating for xcex3-phase transition satisfy the following formula (3) in the temperature range of 580xc2x0 C. or above:
Vxe2x89xa7xe2x88x9250T+275xe2x80x83xe2x80x83(3)
Further, a method of manufacturing the steel wire according to the present invention is characterized by comprising the steps of: shaving a steel wire of pearlite structure containing 0.7-1.0 mass % of C and 0.5-1.5 mass % of Si, patenting the resultant steel wire, and drawing the patented steel wire. This method of manufacture can produce the steel wire of the present invention without resorting to a quenching and tempering process, and can produce a steel wire having a high heat resistance and fatigue strength at low cost. Further, it is preferable to process the resultant drawn steel wire through a strain relief annealing in 350-450xc2x0 C. In this connection, the working ratio of drawing may preferably be kept above 80%.
Hereinafter, the aforementioned features of the present invention will be discussed further in detail.
Chemical Composition
C: The lower limit of the C content was determined based on the fatigue strength, while its upper limit was determined based on the wire drawability.
Si: Si is a chemical element essentially required for improvement of heat resistance. With its content lower than the previously mentioned lower limit no sufficient heat resistance will be achieved, while the resultant steel wire become susceptible to surface flaws if the Si content is higher than its upper limit.
Mo: With an Mo content lower than its lower limit described above it will have a smaller effect on the improvement in the heat resistance and fatigue strength of the steel wire, while its content exceeding the upper limit will elongate the time required for patenting, resulting in a lowered productivity.
Mn: Mn is added for improving the quench hardenability of steel wire. Mn content exceeding the upper limit tends to increase segregation and lowers wire drawability.
Cr: The aforementioned upper limit was determined, because a longer patenting time become required when the Cr content exceeded that level.
Shaving
A purpose of the shaving process is to remove a low hardness layer on the surface of steel wire. The fatigue properties are improved by removing those outer layers having a micro-Vickers hardness at least 50 lower than that of the inner portion of steel wire.
Strain Relief Annealing
The strain relief annealing process is applied at 350-450xc2x0 C. for improving the fatigue properties of spring. An annealing temperature below the lower limit has only a little effect on fatigue properties improvement, the strength and fatigue strength of wire both decrease if the annealing temperature exceeds its upper limit. A preferable annealing time may be about 20 minutes in view of effects and productivity.
Proeutectoid Ferrite
A steel material having a high Si content as in the steel wire according to the present invention has a characteristic of tending to cause proeutectoid ferrite precipitation, which adversely affects on the fatigue properties of steel wire. Keeping the proeutectoid ferrite content below 5 vol. % is effective in improving greatly the fatigue properties and heat resistance of steel wire.
Cementite Morphology
The shape of cementite particles also has an important on the fatigue properties and heat resistance of steel wire. This is because unlike the heat resistance at 450xc2x0 C. or above in the prior art parallel wire a satisfaction of the foregoing formula (1) is desirable for sufficient fatigue properties and heat resistance in the temperature range of 100-200xc2x0 C. according to the present invention.
Relation Between Chemical Composition and Cooling Rate
The aforementioned relation between the chemical composition and the cooling rate after heating for xcex3-phase transition satisfying the foregoing formulas (2) and (3) is required because a steel wire having a metal structure that satisfy the aforementioned requirements for proeutectoid ferrite and cementite particle shape.
[2] Further, the present invention provides a steel wire comprising a pearlite structure plastically worked and containing 0.75-1.0 mass % of C and 0.5-1.5 mass % of Si, wherein cementite particles with the size of 5-20 nm in width are arranged substantially alternately with cementite particles with the size of 20-100 nm in width, said cementite particles of said two different width ranges both having a thickness of 5-20 nm. This steel wire, even if in the form of a piano wire, has at around 200xc2x0 C. a heat resistance substantially equivalent to that of an OT wire. Therefore, it can be used for valve springs of automobile engines and the like.
This steel wire may further contain at least one of Mo and V in total content of 0.05-0.2 mass %, and may also further contain 0.01-0.03 mass % of Al.
Further, it is desired that semicircular stains would not be observed at the interfaces between ferrite and cementite particles as viewed on a transmission electron micrograph.
Furthermore, it is desired that the thickness A1 of cementite particles with the size of 20-100 nm in width and the thicknesswise length A2 of those portions of adjacent cementite particles with the size of 5-20 nm in width contacting the former cementite particles 20-100 nm wide satisfy a relation expressed by the following formula:
0.3 less than A2/A1 less than 0.95
According to the present invention, the most suitable method to produce the steel wire just described above comprises plastically cold-working a steel wire material of containing 0.75-1.0 mass % of C, 0.5-1.5 mass % of Si so that a 0.7 or higher true strain is obtained, said step of plastically cold-working being at least one of drawing, rolling, roller die drawing and swaging, wherein the true strain in one cycle of cold working is kept in the range of 0.1-0.25, the direction of the steel wire is reversed front end rear in the course of working, and the resultant plastically cold-worked steel wire is subsequently heat-treated at 230-450xc2x0 C. This method of manufacture can produce the steel wire according to the present invention having a high heat resistance at a low cost. More preferably, the torsion of the steel wire in the aforesaid plastically cold-working process may be kept within 15xc2x0 per 100 mm of steel wire length.
Now, the aforementioned features of the present invention will be discussed further in detail.
C: 0.75-1.0 Mass %
With a C content lower than 0.75 mass %, the steel wire will have a low strength as well as a low heat resistance. While, with a C content exceeding 1.0 mass %, the plastic working will become difficult as the Si content is increased.
Si: 0.5-1.5 Mass %
With an Si content lower than 0.5 mass %, the steel wire will have a low heat resistance, while the plastic working will become difficult if the Si content exceeds 1.5 mass %.
Cementite Particles Shape and Size
If the conditions that cementite particles with the size of 5-20 nm in width are arranged substantially alternately with cementite particles with the size of 20-100 nm in width and that the cementite particles of said two different width ranges both have a thickness of 5-20 nm are not maintained, the heat resistance of the steel wire at up to about 200xc2x0 C. will decrease.
Ferrite-cementite Interfacial Strain
The heat resistance of steel wire will decrease remarkably if semicircular-stains are observed at the interfaces between ferrite and cementite particles.
State of Contact between Adjacent Cementite Particles
If the relation between the thickness A1 of cementite particles 20-100 nm wide and the thicknesswise length A2 of those portions of adjacent cementite particles 5-20 nm wide contacting adjacent the former cementite particles 20-100 nm wide falls outside the range defined by the formula: 0.3 less than A2/A1 less than 0.95, the steel wire will have a decreased heat resistance.
Total Mo and V Content of 0.05-0.2 Mass %
If the total content of Mo and V in the steel wire exceeds the above said rage, it will become difficult to obtain the pearlite structure. Specifically, it takes a longer time for transformation, resulting in a remarkable decrease in productivity.
Al: 0.01-0.03 Mass %
An Al content in the aforementioned range is effective in improving the toughness of the steel wire.
Cold Plastic Working
The toughness of steel wire will decrease if the true strain falls outside the range of 0.1-0.25. Further, reversing the direction of the steel wire in the course of working process can additionally improve the toughness the steel wire.
Torsion in Working
If the torsion of the steel wire in the aforementioned plastically cold-working process is kept within 15xc2x0 per 100 mm of steel wire length, the heat resistance of the steel will be improved and the shape and size of cementite particles can be stabilized.
[3] Further, the present invention provides a steel wire of pearlite structure containing 0.7-1.0 mass % of C, 0.5-1.5 mass % of Si and less than 0.2 mass % of Cr, wherein a relation given by the following formula (4) is satisfied at 250xc2x0 C. or below:                     γ        ≤                              0.00004            ⁢                          xe2x80x83                        xc3x97            A                    -          0.035          +                      (                                                            (                                      A                    -                    100                                    )                                xc3x97                                  (                                      B                    -                    450                                    )                                            750000                        )                    +                      (                                                            0.015                  xc3x97                  log                  ⁢                                      xe2x80x83                                    ⁢                                      (                                          C                      +                      1                                        )                                                  1.38                            -              0.015                        )                                              (        4        )            
where xcex3 is a residual shear strain (%), A represents a temperature (150xc2x0 C. or above), B represents a shear stress (300 MPa or above), and C represents a time (0.1 hr. or longer), and
wherein a relation given by the following formula (5) is satisfied:
TDF greater than 200/xcfx84xe2x80x83xe2x80x83(5)
where
xcfx84: a shear stress of 200 MPa or above,
TDF: a time elapsed before fracture occurrence (hr.) as tested under said shear stress in a 20% ammonium thiocyanate solution at 50xc2x0 C.
This steel wire according to the present invention has a high the thermal permanent set resistance and high delayed fracture properties. Particularly, the steel wire is excellent in the thermal permanent set resistance at around 200xc2x0 C., and best suited for a spring for automobile engines and associated peripheral parts thereof.
In this connection, it may be preferable that the steel wire further contain 0.01-1.0 mass % of Ni and/or at least one of 0.01-0.15 mass % of Ti and 0.01-0.15 mass % of V.
It may also be desirable to keep the lattice distorsion of the ferrite in the pearlite structure in the range of 0.05-0.2%.
As to a method for manufacturing the above-mentioned steel wire according to the present invention, a die angle of the die used in drawing may be set at 10-8xc2x0 in the method of manufacturing a steel wire comprising a patenting step followed by a drawing step. Further, it is desired that the bearing length of a die having a diameter of d be in the range of d/4-d/5.
Now, the aforementioned features of the steel wire according to the invention will be discussed further in detail.
Formula (4)
When the steel wire is used as a spring, particularly, as a heat-resistant spring, the following three factors will have an important meaning in respect of its working environment: (1) working temperature, (2) working time, and (3) working stress. As will be apparent from experimental examples to be described herein later, it has been found that satisfying the foregoing formula (4) is effective in improving the heat resistance of the steel wire. For reference""s sake, though the conditions of the formula (4) are satisfied with an Sixe2x80x94Cr steel oil tempered steel wire or the like, such-a steel wire is not only expensive, but unable to satisfy the succeeding formula (5) and inferior in delayed fracture properties.
Formula (5)
Another important properties for spring include superiority in delayed fracture properties. As will be shown in experimental examples to be described herein later, satisfying the formula (5) is very effective in improving the delayed fracture properties later. For evaluating the delayed fracture properties the steel wire, a stress condition has an important meaning as its working environment. Although heretofore the delayed fracture properties have been typically evaluated in tensile stress, it is particularly important for a steel wire for spring to evaluate it in terms of torsion stress because such springs are often used in environments involving an application of torsion. Moreover, because of a necessity, in evaluating the delayed fracture, of fixing constant the condition of hydrogen ingress which may cause a delayed fracture, for the evaluation purpose the steel wire specimens were immersed in a 20% ammonium thiocyanate solution at 50xc2x0 C.
C: 0.7-1.0 Mass %
With a C content below 0.7 mass % the steel wire will show a decrease in strength, particularly in fatigue strength, while its content exceeding 1.0 mass % will lowers the drawing workability, thus decreasing the productivity.
Si: 0.5-1.5 Mass %
With an Si content below 0.5 mass % the heat resistance will be decreased, while its exceeding 1.5 mass % will lowers the drawing workability, thus decreasing the productivity.
Cr: 0.2 Mass % or Less
Although the strength may be improved by the addition of Cr, its content exceeding 0.2 mass % will elongate the heat treatment time required for pearlite transformation and result in remarkable reduction in productivity. Here, if the Cr content is in the range of 0.04-0.1 mass %, it is more preferable that the Ni content be xc2xc of the Cr content (mass %) or more but 1.0 mass % or less.
Lattice Distorsion: 0.05-0.2%
If the amount of lattice distorsion is below 0.05%, the steel wire will have a low heat resistance, while if it exceeds 0.2%, such a low material strength will result that fails to satisfy a property required for spring.
Ni: 0.01-1.0 Mass %
A Ni content below 0.01 mass % will result in poor delayed fracture properties. With a Ni content exceeding 1.0 mass % its effect on improvement of the delayed fracture properties will be saturated, only adding to the cost because of expensiveness of nickel. However, in order for this added component to exhibit a sufficient effect on the improvement of both the heat resistance and the delayed fracture properties, it may preferably be contained in an amount in the range of 0.1-1.0 mass %. Further, a Ni content of 0.2-1.0 mass % is more preferable for securing a heat resistance at a temperature range exceeding 200xc2x0 C. for a prolonged period.
At Least One of Ti and V: 0.01-0.15 Mass % Each
If the content of neither of Ti and V is 0.01 mass % or above the steel wire will have poor delayed fracture properties, while if either one is contained in an amount exceeding 0.15 mass %, the steel wire will have a decreased toughness, and become difficult to be used as a spring.
Die Angle: 10-8xc2x0, Bearing Length: d/4-d/5, with d Representing Die Diameter
By thus limiting the die angle and bearing length, the macroscopic distribution of strains introduced in the course of drawing process, particularly, of strains at ferrite-cementite interfaces are uniformized, so that the strains at those interfaces may be readily relieved, while at the same time providing a heat resistance.
[4] Further, the present invention provides a steel wire comprising a pearlite structure and containing 0.7-1.0 mass % of C and 0.5-1.5 mass % of Si, wherein in the pearlite structure the lattice constant a and the lattice distorsion xcex94aLS satisfy a relation given by the following formula:
0.001xc3x97axe2x89xa6xcex94aLSxe2x89xa60.002xc3x97a
The steel wire of the present invention having a pearlite structure of which the lattice constant and lattice distorsions are limited as above can have a remarkably improved fatigue strength.
Here, it is preferable that the steel wire contains Mn and Cr each in an amount of 1 mass % or less. As the most suitable applications, these steel wires according to the present invention may be further worked into springs or twisted to be used as springs for automobile parts and components requiring a high fatigue strength or as reinforcing steel wires including stranded PC steel wires, control cables, steel cords, parallel wires, etc. In worked into a spring, it is preferred that the resultant spring have a surface residual stress comprising a tensile stress of 100 MPa or less or a compression stress. A preferable range of the previously mentioned lattice constant a may be 2.8670-2.8705 xc3x85.
Further, the present invention provides a steel wire comprising a pearlite structure and containing 0.7-1.0 mass % of C and 0.5-1.5 mass % of Si, wherein in the pearlite structure the lattice constant a and the lattice distorsion xcex94aLS satisfies a relation given by the following formula:
0.0025xc3x97axe2x89xa6xcex94aLSxe2x89xa60.0045xc3x97a
In this case, it is preferable that the lattice constant a is in the range of 2.8670-2.8710 xc3x85.
Further, according to the present invention, the most suitable method to produce the steel wire just described above comprises the steps of: cold-working a steel material of pearlite structure containing 0.7-1.0 mass % of C and 0.5-1.5 mass % of Si, so that the resultant steel wire has a lattice constant a1 and lattice distorsion xcex94aLS1 satisfying the following formula (1) after the cold-working process:
0.0025xc3x97a1xe2x89xa6xcex94aLS1xe2x89xa60.0045xc3x97a1xe2x80x83xe2x80x83; (1)
and heat-treating the resultant steel wire, so that the lattice constant a2 and the lattice distorsion xcex94aLS1, thereof satisfy the following formula (2):
0.001xc3x97a2xe2x89xa6xcex94aLS2xe2x89xa60.002xc3x97a2xe2x80x83xe2x80x83(2)
Here, it is preferable that the steel wire contains Mn and Cr each in an amount of 1 mass % or less. The cold-working process includes wire drawing, roller die drawing, swaging, a rolling, forging and so on. In addition, the a, may preferably be in the range of about 2.8670-2.8710 xc3x85, the a2 in the range of 2.8670-2.8705 xc3x85. By the cold working, a moderate strain is introduced so as to adjust the strength to a reasonable level, and the subsequent heat treatment relieves the strain moderately, so that microscopic defects may be prevented from concentrating at limited points in order to eliminate or minimize origins of fatigue fracture and thereby to improve fatigue properties. In this connection, the prior art steel wires have typically had a lattice constant a3 in the range of 2.8665-2.8710 xc3x85 and a lattice distorsion xcex94aLS3 in the range of 0.001xc3x97a3-0.0045xc3x97a3 after cold working. Further, the prior art steel wires have typically had, after heat treatment, a lattice constant a4 in the range of 2.8665-2.8695 xc3x85 and a lattice distorsion xcex94aLS4 of 0.0015xc3x97a4 or above, showing a low fatigue strength.
In this context, as to the conditions of drawing (cold working) after the patenting process, (1) the smaller the die approach angle, (2) the smaller the working ratio and (3) the smaller the drawing angle are, the smaller the variation in lattice distorsions becomes. In addition, as to the conditions of heat treatment after the drawing process, the higher the heat treatment temperature is, the smaller the variation in lattice distorsions is. Further, (1) the lattice constant increases with the Si content, (2) the variation in lattice constants increases as the reduction ratio of cold-working decreases, and (3) the variation in lattice constants increases with the heat treatment temperature.
As will be shown in experimental examples to be described hereinafter, it has been found that limiting the lattice constant and the lattice distorsion as mentioned above is effective in remarkably improving the fatigue properties of the steel wire. That is to say, the inventors have for the first time revealed a correlation between lattice distorsion and fatigue and found out that by controlling the lattice distorsion within a proper range such defects as to cause fatigue can be eliminated and the fatigue properties can be improved.
These lattice constants per se have been observed with the prior art steel wires (, however, without being controlled). However, it has not been so far practiced nor proposed to specifically limit or define the lattice distorsion as falling in a range befitting to the lattice constant. In other words, simply based on an idea that increasing the tensile strength might also increase the fatigue strengths, the following measures have heretofore taken, without being able to improve the fatigue strength: (1) increasing the strength of pearlite (decreasing the patenting temperature), (2) increasing the working ratio of drawing and (3) increasing the material strength by increasing the C content.
Contrary to the aforementioned situations, the inventors have found out that the average amount of strains and the distribution thereof having effect on the improvement of fatigue strength may be controlled. These findings shows that a preferable average amount of strain may be provided by a lattice constant in the range of 2.8670-2.8705 xc3x85 and a preferable strain distribution may be provided by a lattice distorsion xcex94aLS defined as 0.001xc3x97axe2x89xa6xcex94aLSxe2x89xa60.002xc3x97a. These facts indicate that the fatigue properties may not be improved by merely resorting to such factors as patenting condition, working ratio and chemical composition, etc. as in the prior art, and that the fatigue strength is not determined only by the tensile strength of final products.
The lattice constant may be determined by X-ray diffraction. While the lattice distorsion may also be determined by X-ray diffraction, an analysis based on the half-width (width at half-height) of ordinary diffraction peaks and the like is qualitative in nature, and even if the half-width is digitized absolute values resulting therefrom may have a low accuracy, so that it may sometimes be impossible to tell which of two values is larger should their difference be several 10% or less. Then, the inventors have undertaken a series of intensive studies on a methodology that enables these parameters to be evaluated with a high degree of accuracy, and consequently have successfully found out a range of material parameters that can contribute to the improvement of fatigue properties. As contrasted to usual X-ray diffraction methods used heretofore, this method determines the lattice distorsion apart from crystal particle size by calculation based on a so-called Wilson method.
First, the lattice distorsion will be discussed. The lattice distorsion will be produced by uneven or non-uniform deformation, rotation, displacement, and working, etc. of unit cells occurring internally of crystals and, microscopically, are caused by point defects and dislocations, etc. Since a unit cell has a size that may be larger or smaller than ideal size of a unit cell involving no strain, there will remain a stress such as a tension or compression. When measuring the lattice size in a material involving such a lattice distorsion by a X-ray diffraction method, its diffraction peak will not become sharp accompanied by a broad width. The extent of the strain may be roughly determined by evaluating the half-width of the diffraction peak (measuring the width of the peak at a height half the peak height).
However, this width may be broadened due to such factors as intrinsic characteristics of the instruments used and crystal particle size (X-ray crystallographic particle size) in addition to the unit cell size. Therefore, in order to evaluate the variation in unit cell size correctly, these factors must be separated one from another. For this purpose, the lattice distorsion is measured accurately.
Now, description will be made on a method for measuring the lattice distorsion. This is a method that is used often for evaluation of ceramics or the like materials. Half-widths of several diffraction peaks are determined, and then the lattice distorsion and crystal particle size are calculated independently of other factors by a so-called Wilson method. Several diffraction peaks are measured, and half-width (integration width) of each peak is determined. In the instant example, 5 diffraction peaks of 110, 200, 211, 220, and 311 are measured. Instrument parameters are calibrated using a half-width of one and the same diffraction peak for a reference sample (a pure iron powder in the instant example), and then a half-width to be affected only by lattice distorsion and crystal particle size is determined. (xcex942xcex8)/(tan xcex8oxc2x7sin xcex8o) is plotted as ordinate against (xcex942xcex8)xc2x72/tan2 xcex8o as abscissa, and the intercept of the plotted locus is determined. Square root of the resultant intercept is divided by 4 to give a lattice distorsion value intended here. (Expansion of the half-width due to crystal particle size is approximated by a Cauchy function, and expansion due to lattice distorsion by a Gauss function.)
It is not always necessary to use 5 diffraction peaks. Further, while it is not always necessary to use the same diffraction peaks as those used in the instant case, accuracy increases as the number of diffraction peaks increase. For evaluation, a value indicating the state of strain distribution is used, with the value being given in an absolute number (or as percentage). Here, xcex942xcex8 is a half-width (integration width) in xe2x80x9cradiansxe2x80x9d, and xcex80 a diffraction angle in xe2x80x9cdegreesxe2x80x9d. By controlling the lattice distorsion with respect to a given C content and Si content based on such an evaluation method as described above, it becomes possible to achieve improved fatigue properties of steel wire that have not been achievable with such a usual evaluation based on the half-width of X-ray diffraction peak as used heretofore.
In the above-described steel wire and method for manufacturing the same according to the present invention, the steel wire is limited in respect of chemical composition and metal structure thereof based on the grounds set force immediately below:
C (0.7 mass % or more, up to 1.0 mass %) is the most effective element to increase the strength of steel wire. With a content less than 0.7 mass % no sufficient strength can be obtained, while its content exceeding 1.0 mass % will bring about a segregation problem, resulting in an impracticability.
Si (more than 0.5 mass %, up to 1.5 mass %) acts basically as a deoxidizer, and is required for decreasing the content of nonmetallic inclusions. An Si content more than 0.5 mass % shows a great effectiveness of strengthening a solid solution, thereby further improving the fatigue properties.
Like Si, Mn also acts as a deoxidizer. With an Mn content above 1 mass %, the hardenability is increased and a longer time is required for pearlite transformation, thus resulting in decreased productivity.
While Cr is effective in increasing the strength, its content may preferably be 1% or less because its content exceeding the upper will increase the hardenability like Mn.
According to the present invention, a pearlite steel is used because it provides a good balance between strength and toughness in the drawing process.